Searching for the differences... Trawl cod-end with aquarium has entered the deck.
Deep-water ribbonworm (Nemertea)
Date:June 23, 2004
Author: Tone Falkenhaug and Henrik Søiland (Institute of Marine Research, Norway)
When a trawl or net comes onboard, scientists gather on the trawl deck, curious about the content of the catch. You may wonder what they are searching for in these nets. Bizarre species from the deep, or possibly even “new” previously unknown species would certainly be highlights of the day.
However, an important issue during this cruise is also to search for the differences: In order to find out how the ecosystem along the Mid Atlantic Ridge is structured, scientists are looking for differences in the catches between either side of the ridge, differences between night and day, between surface and the deep, or differences between northern and southern stations.
So far, sampling has been made at stations situated north of, and within, the Sub Polar Front. This front is the boundary between the low saline and cool water in the subpolar gyre to the north and the warmer water in the subtropical gyre to the south. Crossing this front gives us an excellent opportunity to study differences in species composition between different water masses. Our first station, south of the Sub Polar Front was reached early this morning. Superstation 22 is situated east of the ridge (50 43 N; 27 31 W) with 3640 m bottom depth.
The zooplankton sample from this station contained a new assemblage of organisms, different from what was found on previous stations: Larval plankton, small medusa, sea butterflies (Pteropoda), fish larvae and hundreds of fish eggs. Fish larvae and fish eggs are sorted out from these samples in order to study development and egg buoyancy. The basic knowledge of the early life history of many deep-sea species is scarce, and these studies will improve our knowledge on egg- and larval morphology, vertical distribution and advection.
The Åkra trawl was towed at 2500 m depth, and samples were taken in three different depth strata, with a multisampler attached to three separate cod-ends. The samples contained several new species records for this cruise: The large 30 cm deep-water ribbonworm (Nemertea, figure) is predatory on other invertebrates, which it captures with a unique and remarkable eversible proboscis. Three species of cephalpods, among them a warm water species of “hooked squid” (Onychoteuthis) appeared for the first time. Also in the catch was the unusual deep-water spookfish (Opisthoproctus grimaldii), with tubular eyes pointed upwards. This enables these fishes to locate prey under extremely low light conditions, silhouetted against the light above.